The present invention relates to an image pickup system using an electronic image pickup device for obtaining electronic images.
There are available digital cameras or other like image pickup systems wherein an image obtained through an image-formation optical system is converted by an electronic image pickup device such as a CCD into image information, and the image is reproduced for observation on a monitor or printer on the basis of that image information. Currently available digital cameras are of various types from the inexpensive low-end type to the high-end type equipped with sophisticated functions.
Of these, the low-end types are designed to be compatible with a human's general range of field of view, and have a zoom type image-formation optical system capable of covering as far as a medium or telephoto range.
In models corresponding to the high-end types, on the other hand, making use of interchangeable lenses designed for silver-salt cameras is now in vogue, with CCDs located at image positions.
A high-end digital camera of the type with a zoom lens of high magnification incorporated as an image-formation optical system is also available with an image pickup range where a human's general range of field of view is defined as the wide-angle end.
With a digital camera of the low-end type or the type using a zoom lens of high magnification, however, any ample field of view is not obtained even at the wide-angle end in the photographable angle of view. Thus, such a digital camera is not suitable for photographing extensive, profound subjects such as landscapes.
If a wide-conversion lens to obtain a focal length shorter than that at the wide-angle end is mounted on a taking lens, it may be possible to photograph a wide-angle range. However, this leads to severe degradation of the image-formation capability of an optical system. When a low-end digital camera is used to photograph a wide range of landscapes or the like having high spatial frequencies, it is impossible to provide any satisfactory reproduction of such spatial frequencies because a CCD mounted on the digital camera is insufficient in respect of the number of pixels. In addition, when the image is enlarged and printed to general appreciation size, the resulting image quality is unsuitable for appreciation purposes.
With a high-end digital camera capable of interchanging lenses, a wide extent of subjects may be prima facie photographed when a so-called wide-angle lens or ultra-wide-angle lens is selected from interchangeable lenses. However, these lenses are designed on the premise that they are used with silver-salt films; the angle of incidence of light on the periphery of a CCD is large, ending up with degradation of images.
Thus, when existing digital cameras are used to photograph subjects of high spatial frequencies such as landscapes, it is difficult to obtain clear-cut images all over the range to be photographed. In addition, not only is definition less than satisfactory but also no sufficient image quality is obtained on the periphery of the image-pickup plane. Especially when the image is enlarged and printed to appreciation size, the unclearness of the image becomes noticeable; the quality of the image does not reach that of a silver-salt photograph at all.
JP-A 05-347730 discloses a method for subjecting an image to so-called edge enhancement processing, thereby giving apparent sharpness to the image. Most of image pickup systems now on the consumer-oriented market, too, make use of such edge enhancement processing and so appear to succeed in achieving sharp images. As far as electrical processing is concerning, edge enhancement may be applied to an image with no substantial restrictions. However, an image subjected to unreasonable enhancement processing undergoes too much degradation upon some considerable enlargement. Accordingly, the level of edge enhancement should preferably be reduced as much as possible so as to achieve high-definition images. In this regard, too, the image quality achieved with existing consumer-oriented image pickup systems is still quite inferior to that of silver-salt photographs at all.